A variety of devices are known for moving fluids such as water, including a variety of pumps and propeller designs. In the field of fluid propulsion, motor-driven propellers often used to move a marine vehicle, such as a ship or a submarine, through water. These propellers typically consist of a twisted airfoil shape, similar to those used as aircraft propellers, and are often only partially submerged in water when operated.
An example of a propeller having a twisted airfoil shape is described in U.S. Pat. No. 4,767,278. One problem associated with the twisted airfoil shape of this type is that fluid is expelled laterally away from the axis of rotation as the propeller is rotated. The kinetic energy of this centrifugal loss does not serve to propel the vehicle forward, because the fluid is not impelled rearward to any degree, but mainly radially away from the axis of rotation. Therefore, propellers of this type are not efficient and result in wasted energy and resources used to drive the propeller.
Another problem associated with the twisted airfoil shape is cavitation, which often occurs at various points over the length of the propeller as it is rotated at different speeds. Cavitation stems from formation of vapor bubbles in a region where the pressure of the liquid falls below its vapor pressure, and can cause a great deal of noise, damage to the propeller, and vibration, as well as a loss of efficiency.
Generally, prior art propellers in various forms have used the same basic shape and design for over a hundred years, and these designs are still affected by serious problems, e.g., cavitation at different points at certain speeds, with a consequent erosion and vibration of its blades, centrifugal loss of fluid, general inefficiency due to drag or other factors, and configurations that limit the speed of the vessel.
As an alternate design approach, various screw propellers have been proposed that provide a greater surface contacting the water. For example, U.S. Pat. No. 941,923 to Hoffman discloses a boat with a screw-shaped propeller. Generally these screw propellers suffer from surface area drag, as well as suffering from the same problem of lateral centrifugal fluid loss and large swirling or vortices that squander the kinetic energy imparted to the water.